1. Field of the Invention
The invention relates to a process for treating domestically-grown dates to obtain a processed date having the characteristics of Middle Eastern imported dates. The invention also relates to dates treated by the process.
2. Prior Art
Dates are grown on date palms, Phoenix dactylifera L., over a wide expanse of the semitropical middle latitudes of the eastern and western hemispheres. Dates have been used as a staple food for many years. They are of high energy value and are comprised of approximately two-thirds sugar. The date is a sticky one-seeded fruit or berry, usually oblong in shape, which varies much in shape, size, color, quality and consistency of flesh, according to the variety and the conditions of culture. The variety usually grown in California and Arizona is the Deglet Noor, a semisoft fruit with a non-sticky skin. The fruits are light brown in color but darken on storage. The dates grown in the Middle East are darker than the dates grown in the western United States. The commercial product is dehydrated, usually at a low temperature, to retain maximum flavor quality. The dried fruit is over one half sugar by weight (usually 70 to 80 percent) and contains about 2 percent each of protein, fat and mineral matter. Dates contain a high percentage of sugar either as invert or as sucrose, the level of which is maximum just before ripening. In the dry varieties such as Deglet Noor the sugar is mostly sucrose, while the moist varieties contain invert sugar (glucose and fructose).
Maier, V.P., et al., "Studies on Domestic dates, II. Some Chemical Changes Associated with Deterioration", pp. 322-328, reports that sucrose inversion, the result of invertase action, can be prevented by a mild heat treatment.
Imported dates have a darker appearance, different flavor, and a softer, chewier bite than U. S. domestic dates. The domestic dates have a more fibrous, tougher chew and contain more rag than imported dates. Additionally, imported dates are lower in moisture content than domestic dates and can be packaged without drying to obtain a shelf-stable product. Controlled drying is necessary to obtain a shelf-stable domestic date. A desirable characteristic of domestic dates is their uniform size and shape. The shape is retained during packaging whereas imported dates are typically squashed upon packaging into a box. Domestic dates can basically be described as being lighter in color, less sweet, less flavorful (more fruity), and higher in moisture (therefore of borderline shelf stability at room temperature) then imported dates while domestic dates have a fibrous-type of firm texture. They are also more uniform in size and shape than imported dates.
Imported dates contain nearly all invert sugar whereas the domestic dates contain relatively high levels of sucrose with small amounts of invert sugar. Treatment of domestic dates with enzymes to convert the sucrose into invert sugars would be uneconomical.
The nonenzymic browning reactions are heat-induced dehydration, degradation, and condensation reactions which are accompanied with the development of yellow to brown color, and characteristic flavor. Nonenzymatic browning is usually either a caramelization or a Maillard reaction.
The Encyclopedia Of Food Technology, The AVI Publishing Co., (1974), at p. 138, reports that at varying water concentrations, the browning reactions are qualitatively different. At relatively high temperatures dehydration of the sugar itself will supply the moisture to accelerate the reaction. At lower temperatures, the air humidity is an important factor. At a given temperature, increasing the water concentration will accelerate browning until, by virtue of dilution of reactants, the rate decreases. The optimum water concentration for browning seems to be about 30 percent. The Encyclopedia, at p. 139, also reports that the processing and storage of susceptible products at the lowest possible temperature aids in preventing browning. Adjustment of the moisture content, either to low moisture in some cases, or to dilute solutions in others, may inhibit browning. At times, lowering the pH of the system may effectively inhibit browning during processing, after which the pH can be readjusted to the desired value.
The natural tannins of dates and the enzymatic and Maillard browning which take place during normal storage impart a brown color and a somewhat caramelized flavor. At 140.degree. F. or so a reddish-brown color as well as astringency and off-flavors occur in Deglet Noor dates. Maier, V.P., Metzler, D.M., and Huber A.M., "Effects of heat processing on the properties of dates", Date Growers Inst. Rept., 41, (1964), 8 to 9. Heat processing coupled with moisture control can be used to improve the color, texture and general quantity of Deglet Noor dates, but again attempts to accelerate the process by use of temperatures in excess of 140.degree. F. may result in the development of off-flavors, off-colors and astringency.
U.S. Pat. No. 3,365,309 (Pader et al.) teaches that fruits should not be subjected to high temperature processing as it substantially destroys or weakens tissue structures.
U.S. Pat. No. 4,364,968 (Waitman et al.) immerses grapes or other fruit into an aqueous solution of a hydrophillic carbohydrate (e.g., fructose, high fructose corn syrup or honey corn syrup) and heats together the grape and carbohydrate solution at 200.degree. F. The heating infuses the carbohydrate into the fruit and melts any surface wax. The excess solution is removed and fruit is dried to a moisture level below 30 percent. Then, if grapes are used, the dried grapes are converted to simulated raisins by storage at a controlled humidity and a high temperature which causes a darkening in color. So the darkening is stated to occur after the drying step has been performed.
Caramelization is generally a relatively high-temperature chemical change in the dates which results in flavor and color changes in the date skin and flesh.
U.S. Pat. No. 2,591,213 teaches that the undesirable caramelization of relatively-dry dates (as indicated by the redness in the fleshy part of the date) occurs when the dates are subjected to water vapor at a temperature of 150.degree. F. or higher.
Deglet Noor dates are usually light brown in color. Maier et al., ibid., (1964) 8, reports that dark brown dates are considered to be undesirable. Page 9 of Maier et al. reports that tough dates have invert sugar contents of 25 to 35 percent (dry weight basis), whereas tender dates have invert sugar contents above that.
In U.S. Pat. No. 4,350,711 (Kahn et al.) sugar solutions are infused into fruits such as dates to increase their stability. Such process results in a glaceed fruit such as the fruits used in the production of fruit cakes.